Energy use in a typical sulfate pulp mill, sometimes referred to as a Kraft mill, is a major operating expense and contributes a significant percent of cost of pulp, as much or more than the wood chips used to manufacture pulp. The conventional fuels have been in order of use: natural gas, fuel oil, coal and wood wastes from the forest or other wood processing plants such as sawmills, chipboard plants or plywood plants. Economic factors have affected each of these fuels so as to increase their costs, subjecting the pulp manufacturer to increasing energy costs.
Energy is used primarily as large quantities of thermal energy convertible into steam for use as the primary medium of heat transfer in a pulp mill. Steam is used in the heating of the wood chip digester, in the extraction of processing chemicals from the pulp, for recovering processing chemicals and in the drying of the pulp. Steam is usually generated in a boiler system which consists of a furnace with heat exchanger coils to conduct water through the combustion chamber where it is turned into steam. The steam is then conveyed by pipes to the locations within the pulp mill where it is to be used. In a large 1000 ton per day mill, as much as one million pounds of steam an hour may be required in the manufacturing process. It is this demand for steam and its generation that the process described in this patent addresses.